1. Field of the Invention
The present invention relates to an optical waveguide used with a coherent light in the field of optical information processing or optical measurement, a light wavelength conversion device using the same, a short wavelength light generation apparatus using the same, an optical information processing apparatus using the same, and a method for producing the same. The present invention further relates to a coherent light generation apparatus and an optical system using the same.
2. Description of the Related Art
An optical waveguide has been used as an optical wave controlling means in a wide variety of technical fields, including optical communications, optical information processing, optical measurement, and the like. Particularly, the application of an optical waveguide to an optical wavelength conversion device has been actively studied. Such an optical wavelength conversion device can convert the wavelength of a semiconductor laser device to realize a small-size short wavelength light source.
A conventional optical wavelength conversion device 600 will now be described with reference to FIGS. 24A and 24B. FIG. 24A is a perspective view illustrating an exemplary structure of the conventional optical wavelength conversion device 600. FIG. 24B is a schematic diagram illustrating the conversion of a fundamental wave P1 to a higher harmonic wave P2 through the optical wavelength conversion device 600 of FIG. 24A.
A conventional optical wavelength conversion device as the optical wavelength conversion device 600 of FIG. 24A is disclosed in, for example, Japanese Laid-Open Publication No. 5-273624. In the conventional optical wavelength conversion device 600, a nonlinear deterioration layer is provided in the vicinity of the surface of the optical waveguide so as to achieve an increased efficiency and stabilization of the operation of the optical wavelength conversion device. Referring to FIG. 24A, the optical wavelength conversion device 600 includes an LiTaO3 substrate 601, an optical waveguide 602, a polarization inverted region 603, and a nonlinear deterioration layer 605.
The TM00 mode fundamental wave P1 enters the optical waveguide 602 of the optical wavelength conversion device 600 illustrated in FIG. 24. Then, the TM00 mode fundamental wave P1 is converted into a TM10 mode higher harmonic wave P2 as it propagates through the optical waveguide 602 along which a plurality of the polarization inverted regions 603 are provided periodically. Typically, the nonlinear deterioration layer 605 has a thickness of about 0.45 xcexcm, and the optical waveguide 602 has a thickness of 1.8 xcexcm.
As illustrated in FIG. 24B, the TM10 mode higher harmonic wave P2 has two peak outputs of about the same amplitude, on the +E side and on the xe2x88x92E side, respectively. The efficiency of the wavelength conversion operation is increased by increasing the overlap between the TM10 mode higher harmonic wave P2 and the TM00 mode fundamental wave P1 each having an intensity distribution as illustrated in FIG. 24B. Moreover, by choosing TM10 mode for the mode of the higher harmonic wave P2 obtained by the wavelength conversion, it is possible to distribute the power density. Thus, it is possible to suppress optical damages even when the higher harmonic wave output is high.
Another conventional optical wavelength conversion device is disclosed in, for example, Japanese Laid-Open Publication No. 4-254834, in which a high refractive index layer having a higher refractive index than that of the optical waveguide is provided on the optical waveguide. FIG. 25A is a perspective view illustrating such a conventional optical wavelength conversion device 640 with a high refractive index layer. FIG. 25B is a schematic diagram illustrating the confinement of the fundamental wave P1 incident on the optical wavelength conversion device of FIG. 25A.
The fundamental wave P1 enters an end surface 645 of a proton exchange optical waveguide 642 provided on an LiNbO3 substrate 641 of the optical wavelength conversion device 640 as illustrated in FIG. 25A. Then, the fundamental wave P1 is converted into the higher harmonic wave P2 as it propagates through the optical waveguide 642 along which a plurality of the polarization inverted regions 644 are provided periodically. The higher harmonic wave P2 is output through another end surface 646 of the optical waveguide 642.
The device illustrated in FIG. 25A further includes a TiO2 high refractive index layer 643 on the surface of the optical waveguide 642. The refractive index of TiO2 used in the high refractive index layer 643 is greater than that of the proton exchange optical waveguide 642. When such layer 643 having a high refractive index is provided on the optical waveguide 642, the confinement of the fundamental wave P1 into the optical waveguide 642 (strictly speaking, the polarization inverted regions 644 therein), as illustrated in FIG. 25B, thereby increasing the efficiency of the wavelength conversion operation of the optical wavelength conversion device 640.
Still another conventional structure for an optical waveguide of an optical wavelength conversion device is disclosed in, for example, Japanese Laid-Open Publication No. 1-238631. Japanese Laid-Open Publication No. 1-238631 discloses an optical wavelength conversion device which employs a ridge-shaped optical waveguide structure in order to increase the light confinement into the optical waveguide.
It has also been suggested in the prior art to provide on the optical waveguide a high refractive index cladding layer having a refractive index higher than that of the optical waveguide, so as to match the phase of the fundamental mode wave propagating through the optical waveguide with that of the higher mode wave, thereby increasing the overlap between the guided optical waves, and thus realizing an efficient wavelength conversion.
Japanese Laid-Open Publication No. 9-281536 discloses another conventional wavelength conversion technique, in which a proton exchange optical waveguide is provided through a proton exchange process and an annealing process, and a second proton exchange region is further provided, so as to convert a fundamental wave propagating through the optical waveguide into a second harmonic wave having a higher guide mode.
The problems associated with the above-described conventional optical wavelength conversion devices will now be described.
The conventional optical wavelength conversion device 600 illustrated in FIG. 24A, the nonlinear deterioration layer 605 is provided in the vicinity of the surface of the optical waveguide 602, as described above, so as to match the phase of the TM00 mode fundamental wave P1 and that of the TM10 mode higher harmonic wave P2, thereby increasing the efficiency of the wavelength conversion operation and improving the resistance against optical damages. With this conventional structure, the emitted TM10 mode higher harmonic wave P2 has an intensity distribution with two peaks on the +E side and the xe2x88x92E side, respectively. Therefore, the focusing characteristics may be low.
With this conventional structure, the output higher harmonic wave P2 has two peaks having about the same amplitude, so as to distribute the power density of the higher harmonic wave P2 among the higher mode peaks. In order to focus the higher harmonic wave output having such an intensity distribution, it is necessary to provide a special optical system. Such an optical system is complicated and difficult to downsize. Moreover, in order to focus the beam to the diffraction limit, it is necessary to shape the beam significantly, thereby reducing the efficiency of the output light to 50% or less.
Moreover, the nonlinear deterioration layer 605 does not act to increase the confinement of the fundamental wave P1. Therefore, it is not possible to increase the power density of the fundamental wave P1, thereby limiting the improvement in the efficiency.
In the conventional structure illustrated in FIG. 25A, the high refractive index layer 643 having a higher refractive index than that of the optical waveguide 642 is provided so as to increase the confinement into the optical waveguide 642, thereby increasing the efficiency. However, the conventional structure has a problem in that it uses a dielectric film having a high refractive index as the high refractive index layer 643 on the optical waveguide 642. Specifically, the high refractive index layer 643 on the optical waveguide 642 has a substantial influence on the effective refractive index of the optical waveguide 642, whereby a high precision is required for the uniformity in the thickness across the entire area of the optical waveguide 642. For example, in the case of an optical wavelength conversion device, the phase matching condition for the entire length of the optical waveguide is strictly dependent upon the effective refractive index of the optical waveguide. Therefore, if there is any change in the effective refractive index of the optical waveguide, the wavelength conversion efficiency considerably decreases. Thus, a strict uniformity is required for the thickness of the high refractive index layer 643.
In the above-described structure, a material different from that of the substrate 641 is deposited on the surface of the optical waveguide 642, whereby waveguide losses easily occur at the interface between the optical waveguide 642 and the high refractive index layer 643. Moreover, when a strain is introduced to the optical waveguide 642 due to a difference in the expansion coefficient between the high refractive index layer 643 and the substrate 641, the effective refractive index of the optical waveguide 642 will have a distribution along the propagation direction.
Furthermore, it has been found in the art that the propagation loss of the light propagating through the optical waveguide 642 due to the high refractive index layer 643 is problematic.
Generally, the propagation loss along the optical waveguide, which deteriorates the characteristics of the optical wavelength conversion device, is classified into the loss of higher harmonic wave and the loss of fundamental wave. It has been found in the art that the conventional high refractive index layer 643 made of a dielectric has less propagation loss for fundamental wave, but that has substantial propagation loss for the higher harmonic wave having shorter wavelengths. For example, an actual experiment has shown that dielectric films having various high refractive indices have substantial propagation loss of several decibels per cm for the higher harmonic wave in the 400 nm wavelength range. It was found that the wavelength conversion efficiency of the optical wavelength conversion device had been reduced by a factor of 2 or more.
Moreover, with the conventional structure having the nonlinear deterioration layer, since the nonlinear deterioration layer does not has a function of increasing the confinement of the fundamental wave, it is not possible to increase the power density of the fundamental wave, thereby limiting the improvement in the efficiency.
The other conventional optical wavelength conversion device aims to increase the overlap between the fundamental wave and the second harmonic wave. However, the guide mode has a substantial distribution through the optical waveguide due to the difference in the refractive index diffusion between the fundamental wave and the higher harmonic wave, thereby limiting the increase in the overlap between the modes, and it is difficult to significantly improve the wavelength conversion efficiency. Moreover, since the fundamental wave and the second harmonic wave do not overlap with each other over a large area, it is difficult to improve the resistance against optical damages.
With the optical wavelength conversion device as that disclosed in Japanese Laid-Open Publication No. 1-238631, which employs a ridge-shaped optical waveguide in order to improve the wavelength conversion efficiency, the wavelength conversion efficiency is improved by increasing the power density relying upon the confinement effect of the optical waveguide. However, for the fundamental wave, the direction in which the confinement effect can be increased by the ridge-shaped optical waveguide is limited to the lateral direction (the ridge width direction), and the confinement in the depth direction is not improved. Therefore, the overlap between the fundamental wave and the second harmonic wave (particularly, the increase in the overlap in the depth direction), which has the most substantial influence on the wavelength conversion efficiency, cannot be increased by such a ridge structure. Therefore, with the conventional structure, it is difficult to substantially increase the wavelength conversion efficiency.
Furthermore, the overlap between the fundamental wave and the second harmonic wave cannot sufficiently be increased for a loaded-type optical waveguide, as for the ridge structure.
With any of the conventional structures, it is possible to produce blue light of about 10 mW, but it is difficult to obtain a stable output over a long time for an output equal to or greater than 10 mW in view of the resistance against optical damages.
Another conventional optical wavelength conversion device employs a high refractive index cladding layer provided on the optical waveguide so as to realize an efficient wavelength conversion. In this conventional structure, a cladding layer having a refractive index higher than that of the optical waveguide is provided on the surface of the optical waveguide. For the cladding layer, a dielectric film having a high refractive index or a layer provided through an ion exchange process is used. With this structure, it is possible to obtain a high wavelength conversion efficiency and a desirable optical damage resistance. However, it has newly been found that the output becomes unstable over a long period of time.
An optical wavelength conversion device employing a nonlinear optical effect has been used in many different fields as it allows for an increase in the coherent light generation range through the optical wavelength conversion, e.g., second harmonic wave generation, parametric generation, sum frequency generation, difference frequency generation, and the like. Particularly, with an optical wavelength conversion device employing an optical waveguide, it is possible to realize a high power density by the light confinement effect, thereby providing advantages such as a long interaction length. These advantages allow for an efficient wavelength conversion. Thus, may optical devices have been proposed in the art as optical wavelength conversion devices employing an optical waveguide.
One major problem associated with the optical wavelength conversion device employing an optical waveguide is the possible optical damage. The optical damage as used herein refers to an optically induced change in the refractive index. More specifically, in an area where the light confinement is strong, such as along the optical waveguide, the optical power density is high, whereby the refractive index along the optical waveguide may be varied by the light propagating therethrough (optical damage). When there occurs such an optical damage, the phase matching state is varied by the change in the refractive index, thereby varying the output of the optical wavelength conversion device. This has been a problem which hinders the increase in the output of the optical waveguide-type optical wavelength conversion device. Particularly, the occurrence of the optical damages is significant when the substrate is made of a material such as LiNbO3 or LiTaO3, which has a large nonlinear optical constant, in which case optical damages occur for an output of several milliwatts to several ten milliwatts. In order to reduce such optical damages, it is effective to reduce the amount of impurity contained in the crystal and to reduce the propagation loss along the optical waveguide.
To address the problem of optical damages, it has been proposed in the art to dope an LiNbO3 substrate with a metal element such as Zn, Mg, Sc or In so as to improve the optical damage resistance of the substrate itself. According to this method, the optical damage resistance of the crystal itself which forms the substrate can be increased by an order of magnitude by adding about 5 mol of a metal material to the substrate.
In order to improve the efficiency of the wavelength conversion from a fundamental wave to a higher harmonic wave in the case of an optical wavelength conversion device using an optical waveguide (an optical waveguide-type optical wavelength conversion device), it is necessary to increase the power density of the fundamental wave propagating through the optical waveguide, and to increase the electric field distribution overlap between the fundamental wave and the higher harmonic wave after the conversion.
As an optical wavelength conversion device using an optical waveguide, there has been proposed in the art an optical wavelength conversion device 160 using a plurality of optical waveguides as illustrated in FIG. 26. In this optical wavelength conversion device 160, two adjacent optical waveguides 151 are provided on the surface of an LiNbO3 substrate through a proton exchange process so as to match the phase of a fundamental wave Pw 152 which propagates through the two optical waveguides 151 and that of a higher harmonic wave P2w 153 which radiates through Cherenkov radiation. The central portion extending between the two optical waveguides 151 has not been subjected to a proton exchange process, and thus has a high nonlinear optical effect, thereby allowing for formation of a highly nonlinear optical waveguide. As a result, with the Cherenkov radiation type optical wavelength conversion device, it is possible to increase the conversion efficiency.
Moreover, Japanese Laid-Open Publication No. 3-261924 and Japanese Laid-Open Publication No. 5-188420, for example, disclose an optical wavelength conversion device which utilizes the coupling between a plurality of optical waveguides. Such a conventional structure includes a polarization inverted structure, an optical waveguide through which a fundamental wave propagates, and another optical waveguide through which a higher harmonic wave propagates, which are provided on the surface of the substrate, so that the fundamental wave and the higher harmonic wave propagate through the different optical waveguides, respectively, thereby improving the optical damage resistance and the conversion efficiency.
In connection with the conventional optical wavelength conversion device 160, it has been proposed in the art as described above to add a metal element to the substrate so as to improve the optical damage resistance of the substrate. With this method, however, it is not possible to sufficiently improve the optical damage resistance of the optical waveguide, where the optical waveguide is provided through a proton exchange process, or the like.
Moreover, the conventional optical wavelength conversion device 160 illustrated in FIG. 26 generates a higher harmonic wave which is in a radiation mode, and thus has poor focusing characteristics. Therefore, it is difficult to use such a device in an optical system, or the like, where it is necessary to focus the beam to the diffraction limit. Furthermore, the conditions under which the phase of a fundamental wave, which propagates in a guide mode, is matched with that of a higher harmonic wave, which propagates in radiation mode, hold over a wide range. Therefore, it is difficult to control the higher harmonic propagation mode, and it is difficult to select the phase matching relationship which allows for an efficient wavelength conversion.
Moreover, in the structure as those of Japanese Laid-Open Publication No. 3-261924 and Japanese Laid-Open Publication No. 5-188420, in which a fundamental wave and a higher harmonic wave propagate through different optical waveguides, respectively, there is only a small electric field distribution overlap between the fundamental wave and the higher harmonic wave, which propagates the respectively different optical waveguides. Thus, it is difficult to increase the conversion efficiency.
First, a conventional example in which an optical waveguide with an proton exchange process is employed in order to improve the operating characteristics of an optical wavelength conversion device will be described along with the problems that have been newly found in association with such a conventional example.
A conventional optical wavelength conversion device 300 will be described with reference to FIG. 1.
In FIG. 1, a stripe-shaped optical waveguide 305 is provided in the vicinity of the surface of an X-plate LiNbO3 substrate 301, and a high refractive index layer 310 is provided in the vicinity of the surface of the optical waveguide 305. The LiNbO3 substrate 301 includes a plurality of polarization inverted regions 304 which are provided periodically for phase matching. A fundamental wave 306 enters the optical waveguide 305, is converted into a second harmonic wave 307, and output from the optical waveguide 305.
FIG. 2A is a schematic diagram illustrating the conventional optical wavelength conversion device 300 in which an optical waveguide 2 is provided on a substrate 1, with a high refractive index layer 4 being provided in the vicinity of the surface of the optical waveguide 2. FIG. 2B illustrates the refractive index distribution along the depth direction of the structure illustrated in FIG. 2A. FIG. 2C illustrates the electric field distribution of the fundamental wave and the higher harmonic wave propagating through the structure illustrated in FIG. 2A along the depth direction. Specifically, the fundamental wave having a wavelength of 850 nm enters the optical waveguide 2 in a TE00 mode, and the phase of the fundamental wave is pseudo-matched with that of a TE10 mode higher harmonic wave in the optical waveguide 2.
The optical waveguide 2 is an optical waveguide formed by subjecting a proton exchange region to an annealing process. The annealing process changes the refractive index distribution pattern of the proton exchange region from a step pattern to a graded pattern and reduces the proton density. The high refractive index layer 4 has been provided through a proton exchange process but has not been subjected to an annealing process. Thus, the high refractive index layer 4 has a step distribution.
In the optical wavelength conversion device 300, the high refractive index layer 4 increases the confinement of the fundamental wave, and also increases the overlap between the fundamental wave in a fundamental mode and the higher harmonic wave in a higher mode, thereby realizing a high wavelength conversion efficiency.
However, various studies on the output characteristics of the conventional optical wavelength conversion device 300 have shown that there are various problems as follows.
First, it has been found that the phase matching wavelength varies over time. This causes the second harmonic wave output to be unstable.
The second problem is a reduced operating lifetime of the device. Specifically, it has been found that the characteristics of the optical wavelength conversion device deteriorate in a high temperature test.
Third, there is a large propagation loss through the optical waveguide.
Each of these problems will be described in detail below.
Regarding the first problem, slight variations in the phase matching state were observed as the output of second harmonic generation was increased. This problem is related to an optical damage occurring at the boundary between the high refractive index layer 4 and the optical waveguide 2. The cause of this problem will be further described in detail below.
The optical damage as known in the prior art is a phenomenon in which the phase matching state is destroyed across the entire optical waveguide, thereby reducing the output of the second harmonic wave. The problem that has been newly found by the present inventors is a phenomenon in which the phase matching wavelength shifts while retaining the phase matching curve. Thus, although the wavelength conversion efficiency is not reduced, the phase matching wavelength varies, whereby the second harmonic wave output gradually decreases if the wavelength of the fundamental wave is fixed. Therefore, a stable output cannot be obtained unless the phase matching wavelength is always set to an optimal value. When the phase matching wavelength varies, the second harmonic wave output becomes unstable because the phase matching wavelength tolerance of the optical wavelength conversion device is as narrow as 0.1 nm or less.
The second problem was found during a life test of the optical wavelength conversion device. Specifically, phenomena such as substantial variations in the phase matching wavelength of the optical wavelength conversion device or deterioration of the wavelength conversion efficiency were observed during a high temperature test at 80xc2x0 C. for several tens of hours. The possible cause of these phenomena was studied. As a result, it has been found that the high refractive index layer 4 is altered during a high temperature test. The high refractive index layer 4 is a proton layer which is not subjected to an annealing process, and has a high proton concentration and a step-shaped proton concentration distribution pattern. Thus, the proton concentration is high and substantially different from that in the surrounding regions, thereby resulting in a large thermal diffusion constant. Moreover, it has been found that the thermal diffusion of the proton exchange region of the high refractive index layer 4 has a substantial influence on the characteristics of the optical waveguide 2, whereby the characteristics of the optical wavelength conversion device deteriorate due to the alteration of the proton exchange region of the high refractive index layer 4 during a high temperature test.
The third problem is primarily related to the first proton exchange region. In an optical waveguide structure with the high refractive index layer 4 being provided on the surface thereof, the light confinement is increased, whereby the propagation loss along the optical waveguide 2 is likely to increase. Moreover, the propagation loss along the optical waveguide 2 influences the characteristics of the first proton exchange region which has the greatest electric field in the electric field distribution. A study conducted by the present inventors has shown that there is a close relationship between the proton concentration of the first proton exchange region and the propagation loss along the optical waveguide.
According to one aspect of this invention, an optical waveguide includes: a nonlinear optical crystal; a first ion exchange region provided in the vicinity of a portion of a surface of the nonlinear optical crystal; and a second ion exchange region provided in the surface of the nonlinear optical crystal. The second ion exchange region covers a greater area of the surface than an area covered by the first ion exchange region. The second ion exchange region includes a region having an extent of 0.02 xcexcm to 0.2 xcexcm along a depth direction in which an ion exchange ratio varies along the depth direction.
In one embodiment of the invention, the second ion exchange region includes a region having an extent of 0.02 xcexcm to 0.2 xcexcm along a depth direction in which an amount of change in an ion exchange ratio is 5 xcexcmxe2x88x921 to 50 xcexcmxe2x88x921.
In one embodiment of the invention, the first ion exchange region has an amount of change in ion exchange ratio which is less than or equal to 0.06 xcexcmxe2x88x921.
In one embodiment of the invention, the second ion exchange region includes a region having an extent of 0.03 xcexcm to 0.1 xcexcm along a depth direction in which an ion exchange ratio varies along the depth direction.
In one embodiment of the invention, the second ion exchange region has an amount of change in ion exchange ratio of 10 xcexcmxe2x88x921 to 30 xcexcmxe2x88x921.
In one embodiment of the invention, the first ion exchange region has a nonlinear optical constant which is equal to or greater than 90% of that of the crystal, and the second ion exchange region has a nonlinear optical constant which is less than or equal to 60% of that of the crystal.
In one embodiment of the invention, the first ion exchange region has a change of surface refractive index xcex94n which is less than or equal to 0.02 for light having a wavelength of 633 nm, and the second ion exchange region has a change of surface refractive index xcex94n which is equal to or greater than 0.11 for light having a wavelength of 633 nm.
In one embodiment of the invention, the optical waveguide is provided through an ion exchange process in the surface of the nonlinear optical crystal, the nonlinear optical crystal being made of LiNbxTa1xe2x88x92xO3 (0xe2x89xa6xxe2x89xa61), and a molarity percentage of Li in the optical waveguide is equal to or greater than 40 mol %.
In one embodiment of the invention, the optical waveguide is provided through an ion exchange process in the surface of the nonlinear optical crystal, the nonlinear optical crystal being made of LiNbxTa1xe2x88x92xO3 (0xe2x89xa6xxe2x89xa61) and doped with a metal element in the vicinity of the surface, and a molarity percentage Y of Li and a molarity Z of the metal element in the optical waveguide have a relationship of Y+Zxe2x89xa745 mol %.
According to another aspect of this invention, an optical waveguide includes: a nonlinear optical crystal; a first ion exchange region provided in the vicinity of a portion of a surface of the nonlinear optical crystal: and a second ion exchange region provided in the surface of the nonlinear optical crystal. The second ion exchange region covers a greater area of the surface than an area covered by the first ion exchange region. The second ion exchange region has a refractive index higher than that of the first ion exchange region, and the second ion exchange region has a refractive index distribution which is in a step-like graded pattern.
In one embodiment of the invention, the first ion exchange region and the second ion exchange region have different graded patterns obtained by annealing processes at different temperatures, respectively.
In one embodiment of the invention, a depth of the first ion exchange region is expanded by an annealing process to be equal to or greater than 8 times that before the annealing process.
In one embodiment of the invention, a depth of the second ion exchange region is expanded by an annealing process to be equal to or greater than 1.2 times that before the annealing process.
In one embodiment of the invention, a depth of the second ion exchange region is expanded by an annealing process to be equal to or greater than 2 times that before the annealing process.
In one embodiment of the invention, a surface refractive index of the second ion exchange region is substantially equal to that before an annealing process.
In one embodiment of the invention, the step-like graded pattern of the refractive index distribution of the second ion exchange region is provided by an annealing process which satisfies the relationship represented by Expressions 1 and 2 below:
C(k,t)=COxc3x970.5xc3x97{Erf[(h+k)/2/{square root over (Dpxc3x97t)}]+Erf[(hxe2x88x92k)/2/{square root over (Dpxc3x97t)}]}xe2x80x83xe2x80x83Expression 1
(where C(k,t) is an ion exchange concentration, k is a depth (xcexcm), t is an annealing time (hour), CO is an initial ion exchange concentration, Erf[ ] is an error function, h is an initial ion exchange depth (xcexcm), and Dp is an ion diffusion constant by an annealing process (xcexcm2/hour)); and
1.5 less than (h+k)/2/{square root over (Dpxc3x97t)} less than 20.xe2x80x83xe2x80x83Expression 2
In one embodiment of the invention, the nonlinear optical crystal is an LiNbxTa1xe2x88x92xO3 (0xe2x89xa6xxe2x89xa61) crystal.
In one embodiment of the invention, each of an ion exchange process for providing the first ion exchange region and an ion exchange process for providing the second ion exchange region is a proton exchange process.
In one embodiment of the invention, the optical waveguide is provided through an ion exchange process in the surface of the nonlinear optical crystal, the nonlinear optical crystal being made of LiNbxTa1xe2x88x92xO3 (0xe2x89xa6xxe2x89xa61), and a molarity percentage of Li in the optical waveguide is equal to or greater than 40 mol %.
In one embodiment of the invention, the optical waveguide is provided through an ion exchange process in the surface of the nonlinear optical crystal, the nonlinear optical crystal being made of LiNbxTa1xe2x88x92xO3 (0xe2x89xa6xxe2x89xa61) and doped with a metal element in the vicinity of the surface, and a molarity percentage Y of Li and a molarity Z of the metal element in the optical waveguide have a relationship of Y+Zxe2x89xa745 mol %.
According to still another aspect of this invention, an optical waveguide includes: a nonlinear optical crystal; a first ion exchange region provided in the vicinity of a portion of a surface of the nonlinear optical crystal; and a second ion exchange region provided in the surface of the nonlinear optical crystal. The second ion exchange region covers a greater area of the surface than an area covered by the first ion exchange region. The second ion exchange region has an ion concentration higher than that of the first ion exchange region, and the second ion exchange region has an ion concentration distribution which is in a step-like graded pattern.
In one embodiment of the invention, the first ion exchange region and the second ion exchange region have different graded patterns obtained by annealing processes at different temperatures, respectively.
In one embodiment of the invention, a depth of the first ion exchange region is expanded by an annealing process to be equal to or greater than 8 times that before the annealing process.
In one embodiment of the invention, a depth of the second ion exchange region is expanded by an annealing process to be equal to or greater than 1.2 times that before the annealing process.
In one embodiment of the invention, a depth of the second ion exchange region is expanded by an annealing process to be equal to or greater than 2 times that before the annealing process.
In one embodiment of the invention, a surface ion concentration of the second ion exchange region is substantially equal to that before an annealing process.
In one embodiment of the invention, the step-like graded pattern of the ion concentration distribution of the second ion exchange region is provided by an annealing process which satisfies the relationship represented by Expressions 1 and 2 below:
C(k,t)=COxc3x970.5xc3x97{Erf[(h+k)/2/{square root over (Dpxc3x97t)}]+Erf[(hxe2x88x92k)/2/{square root over (Dpxc3x97t)}]}xe2x80x83xe2x80x83Expression 1
(where C(k,t) is an ion exchange concentration, k is a depth (xcexcm), t is an annealing time (hour), CO is an initial ion exchange concentration, Erf[ ] is an error function, h is an initial ion exchange depth (xcexcm), and Dp is an ion diffusion constant by an annealing process (xcexcm2/hour)); and
1.5 less than (h+k)/2/{square root over (Dpxc3x97t)} less than 20xe2x80x83xe2x80x83Expression 2
In one embodiment of the invention, the nonlinear optical crystal is an LiNbxTa1xe2x88x92xO3 (0xe2x89xa6xxe2x89xa61) crystal.
In one embodiment of the invention, each of an ion exchange process for providing the first ion exchange region and an ion exchange process for providing the second ion exchange region is a proton exchange process.
In one embodiment of the invention, the optical waveguide is provided through an ion exchange process in the surface of the nonlinear optical crystal, the nonlinear optical crystal being made of LiNbxTa1xe2x88x92xO3 (0xe2x89xa6xxe2x89xa61), and a molarity percentage of Li in the optical waveguide is equal to or greater than 40 mol %.
In one embodiment of the invention, the optical waveguide is provided through an ion exchange process in the surface of the nonlinear optical crystal, the nonlinear optical crystal being made of LiNbxTa1xe2x88x92xO3 (0xe2x89xa6xxe2x89xa61) and doped with a metal element in the vicinity of the surface, and a molarity percentage Y of Li and a molarity Z of the metal element in the optical waveguide have a relationship of Y+Zxe2x89xa745 mol %.
According to still another aspect of this invention, an optical wavelength conversion device includes: an optical waveguide of the present invention; and a periodic polarization inverted structure. The optical waveguide allows for propagation of a fundamental wave having a wavelength of xcex and a second harmonic wave having a wavelength of xcex/2 therethrough. A refractive index and a depth of the second ion exchange region included in the optical waveguide satisfy a waveguide condition for the second harmonic wave while satisfying a cut-off condition for the fundamental wave.
In one embodiment of the invention, a phase of a fundamental wave in a fundamental mode and a phase of a second harmonic wave in a higher mode are matched with each other in the optical waveguide.
According to still another aspect of this invention, there is provided an optical wavelength conversion device includes: an optical waveguide of the present invention; and a periodic polarization inverted structure. The optical waveguide allows for propagation of a fundamental wave having a wavelength of xcex and a second harmonic wave having a wavelength of xcex/2 therethrough. A refractive index and a depth of the second ion exchange region included in the optical waveguide satisfy a waveguide condition for the second harmonic wave while satisfying a cut-off condition for the fundamental wave.
In one embodiment of the invention, a phase of a fundamental wave in a fundamental mode and a phase of a second harmonic wave in a higher mode are matched with each other in the optical waveguide.
According to still another aspect of this invention, an optical wavelength conversion device includes: an optical waveguide of the present invention; and a periodic polarization inverted structure. The optical waveguide allows for propagation of a fundamental wave having a wavelength of xcex and a second harmonic wave having a wavelength of xcex/2 therethrough. A refractive index and a depth of the second ion exchange region included in the optical waveguide satisfy a waveguide condition for the second harmonic wave while satisfying a cut-off condition for the fundamental wave.
In one embodiment of the invention, a phase of a fundamental wave in a fundamental mode and a phase of a second harmonic wave in a higher mode are matched with each other in the optical waveguide.
According to still another aspect of this invention, there is provided an optical wavelength conversion device, including a optical waveguide group including a plurality of optical waveguides adjacent to one another on a surface of a nonlinear optical crystal, the optical wavelength conversion device being provided for converting in the optical waveguide group an input fundamental wave into light having a different wavelength. The fundamental wave propagates through the optical waveguide group in a single mode, while the converted light propagates through at least one of the optical waveguides of the optical waveguide group in a guide mode, or the converted light propagates through the optical waveguide group in a single mode, while the fundamental wave propagates through at least one of the optical waveguides of the optical waveguide group.
In one embodiment of the invention, the optical waveguide group includes a plurality of optical waveguides having different propagation directions.
In one embodiment of the invention, at least one of the optical waveguides satisfies a cut-off condition for either one of the fundamental wave or the converted light, while satisfying a waveguide condition for the other one of the fundamental wave and the converted light.
In one embodiment of the invention, at least one of the optical waveguides has a propagation constant which is different from those of the other optical waveguides.
In one embodiment of the invention, the optical waveguide group satisfies a single mode propagation condition for either one of the fundamental wave or the converted light, and a phase of the one of the fundamental wave and the converted light is matched with a phase of the other one of the fundamental wave and the converted light, which propagates through at least one of the optical waveguides.
In one embodiment of the invention, the optical waveguide group includes an odd number of optical waveguides and has a symmetric arrangement with respect to a center optical waveguide.
In one embodiment of the invention, the optical waveguide group includes three optical waveguides having a substantially equal propagation direction. A center optical waveguide has a propagation constant which is different from that of the side optical waveguides. A phase of either one of the fundamental wave and the converted light propagating through the optical waveguide group in a single mode is matched with a phase of the other one of the fundamental wave and the converted light propagating through a center optical waveguide of the optical waveguide group.
In one embodiment of the invention, the number of optical waveguides of the optical waveguide group is varied in the vicinity of either an input section and an output section.
According to still another aspect of this invention, there is provided an optical wavelength conversion device, including an optical waveguide which is provided through an ion exchange process in a surface of a crystal of LiNbxTa1xe2x88x92xO3 (0xe2x89xa6xxe2x89xa61), wherein a molarity percentage of Li in the optical waveguide is equal to or greater than 40 mol %.
According to still another aspect of this invention, there is provided an optical wavelength conversion device, including an optical waveguide which is provided through an ion exchange process in a surface of a crystal, the crystal being made of LiNbxTa1xe2x88x92xO3 (0xe2x89xa6xxe2x89xa61) and doped with a metal element in the vicinity of the surface, and a molarity percentage Y of Li and a molarity Z of the metal element in the optical waveguide have a relationship of Y+Zxe2x89xa745 mol %.
In one embodiment of the invention, the metal element is one of Mg, Zn, Sc and In, or a mixture of two or more of Mg, Zn, Sc and In.
According to still another aspect of this invention, there is provided a short wavelength light generation apparatus, including: a semiconductor laser device; and an optical wavelength conversion device of the present invention. A wavelength of light emitted from the semiconductor laser device is converted by the optical wavelength conversion device to a predetermined higher harmonic wave.
In one embodiment of the invention, a guide mode of the higher harmonic wave obtained by the conversion by the optical wavelength conversion device is a higher mode. An intensity distribution of the higher mode of the higher harmonic wave has a plurality of peaks, including a main peak having the maximum intensity and one or more sub peaks. A width of at least one of the one or more sub peaks is smaller than a diffraction limit for the higher harmonic wave.
In one embodiment of the invention, the higher mode of the higher harmonic wave is a first mode.
In one embodiment of the invention, the higher mode of the higher harmonic wave is TE10 mode.
According to still another aspect of this invention, there is provided an optical information processing apparatus, including: a short wavelength light generation apparatus of the present invention; and a focusing optical system. Light having a short wavelength emitted from the short wavelength light generation apparatus is focused by the focusing optical system.
According to still another aspect of this invention, there is provided a method for producing an optical wavelength conversion device, including the steps of: providing a periodic polarization inverted structure in a nonlinear optical crystal; providing a first ion exchange region in the polarization inverted structure; performing a first annealing process for the first ion exchange region; providing a second ion exchange region in a surface of the first ion exchange region; and performing a second annealing process for the second ion exchange region. A first annealing temperature for the first annealing process and a second annealing temperature for the second annealing process are different from each other.
In one embodiment of the invention, the first annealing temperature is equal to or greater than 300xc2x0 C., and the second annealing temperature is less than or equal to 250xc2x0 C.
In one embodiment of the invention, the step of providing the polarization inverted structure includes a step of subjecting the provided polarization inverted structure to a heat treatment at a temperature equal to or greater than 400xc2x0 C.
In one embodiment of the invention, the second annealing temperature is in a range of 130xc2x0 C. to 200xc2x0 C.
In one embodiment of the invention, the second annealing process satisfies the relationship represented by Expressions 1 and 2 below:
C(k,t)=COxc3x970.5xc3x97{Erf[(h+k)/2/{square root over (Dpxc3x97t)}]+Erf[(hxe2x88x92k)/2/{square root over (Dpxc3x97t)}]}xe2x80x83xe2x80x83Expression 1
(where C(k,t) is an ion exchange concentration, k is a depth (xcexcm), t is an annealing time (hour), CO is an initial ion exchange concentration, Erf[ ] is an error function, h is an initial ion exchange depth (xcexcm), and Dp is an ion diffusion constant by an annealing process (xcexcm2/hour)); and
xe2x80x831.5 less than (h+k)/2/{square root over (Dpxc3x97t)} less than 20,xe2x80x83xe2x80x83Expression 2
and wherein a refractive index distribution of the provided second ion exchange region has a step-like graded pattern.
In one embodiment of the invention, the nonlinear optical crystal is an LiNbxTa1xe2x88x92xO3 (0xe2x89xa6xxe2x89xa61) crystal.
In one embodiment of the invention, each of an ion exchange process for providing the first ion exchange region and an ion exchange process for providing the second ion exchange region is a proton exchange process.
According to still another aspect of this invention, there is provided a coherent light generation apparatus, including: an optical wavelength conversion device of the present invention; and a semiconductor laser device. A wavelength of light from the semiconductor laser device is converted by the optical wavelength conversion device.
According to still another aspect of this invention, there is provided an optical system including: a coherent light generation apparatus of the present invention; and a focusing optical system. Light output from the optical wavelength conversion device is focused by the focusing optical system.
The functions of the present invention will now be described.
The present invention provides an optical wavelength conversion device for converting, based on the nonlinear optical effect, the wavelength of a fundamental wave propagating through an optical waveguide into a higher harmonic wave, a parametric wave, a sum frequency wave, a difference frequency wave, and the like. In order to improve the optical damage resistance, the efficiency of the wavelength conversion, and the beam characteristics of the emitted light, the present invention employs a special structure different from those in the prior art for the optical waveguide which is used in the optical wavelength conversion device.
According to Embodiments 7 and 8 of the present invention to be described later, a group of optical waveguides adjacent to one another is provided on the surface of a nonlinear optical crystal. The wavelength conversion from a fundamental wave to a higher harmonic wave, a parametric wave, a sum frequency wave, a difference frequency wave, and the like, is realized by using the optical waveguide group. With such a structure, the phase matching state is controlled so that the optical waveguide group can provide different propagations for the fundamental wave and for the converted light, respectively. For example, as shown in Embodiment 7 to be described below, a fundamental wave can be propagated through the optical waveguide group in a single mode, while a higher harmonic wave or a sum frequency wave is propagated through the optical waveguide in a guide mode. Alternatively, as shown in Embodiment 8 to be described below, a fundamental wave can be propagated through the optical waveguide in a single mode, while a parametric wave or a difference frequency wave is propagated through the optical waveguide group in a single mode. By such an optical waveguide structure, it is possible to reduce the area across which proton exchange, metal diffusion, or the like, occurs, and thus to reduce the damaged area along the optical waveguide surface, thereby significantly reducing the absorption scatterIng along the optical waveguide. Thus, according to the present invention, it is possible to significantly reduce the optical damage which would otherwise occur due to absorption of light having a short wavelength among the fundamental wave and the converted light. Moreover, it is possible to increase the electric field distribution overlap between the fundamental wave propagating through the optical waveguide and the converted light, thereby increasing the efficiency of the wavelength conversion by controlling the propagation mode of one of the fundamental wave or the converted light by the waveguide while controlling the propagation mode of the other by the optical waveguide group. Furthermore, by appropriately selecting the propagation state of the converted light, it is possible to control the aspect ratio of the guide mode and to realize an emission having desirable focusing characteristics.
When the optical waveguide group is formed by a plurality of optical waveguides having different propagation directions, it is possible to provide a distribution in the propagation constant of the optical waveguide group, thereby increasing the tolerance of the optical wavelength conversion device. Moreover, it is possible to produce a tapered waveguide (a waveguide is tapered in either direction along the propagation direction).
When the converted light has a wavelength which is shorter than that of the fundamental wave (e.g., a higher harmonic wave or a sum frequency wave), the width of the optical waveguide can be set so as to satisfy the cut-off condition with respect to the fundamental wave, whereby it is possible to provide a propagation in a single mode through the optical waveguide group as a single waveguide. While the cut-off condition can be defined by the width and the thickness of the waveguide, it is herein defined by the width of the waveguide. Then, the optical waveguide can be set so as to satisfy the waveguide condition for the light having a wavelength obtained by converting the fundamental wave. Thus, as compared to a conventional optical wavelength conversion device with which a higher harmonic wave is guided in a radiation mode, it is possible to easily control the propagation mode of the converted light and to control the phase matching between the fundamental wave and the converted light. When the wavelength of the converted light is longer than that of the fundamental wave (e.g., a parametric wave or a difference frequency wave), the fundamental wave can propagate through any waveguide while the converted light can propagate in a single mode through the optical waveguide group. In such a case, it is possible to easily control the propagation mode of the fundamental wave and to control the phase matching between the fundamental wave and the converted light.
When the converted light has a wavelength which is shorter than that of the fundamental wave (e.g., a higher harmonic wave or a sum frequency wave), the propagation constant of at least one of the optical waveguides can be set to a value different from the propagation constants of the other optical waveguides, whereby the light having the wavelength obtained by converting that of the fundamental wave selectively propagates through that optical waveguide, thereby allowing for improvement of the conversion efficiency and/or stabilization of the output light. Herein, the propagation constant is defined by the width of the optical waveguide. When the wavelength of the converted light is longer than that of the fundamental wave (e.g., a parametric wave or a difference frequency wave), the fundamental wave selectively propagates through the optical waveguide, thereby allowing for improvement of the conversion efficiency and/or stabilization of the output light. In contrast, with a conventional optical waveguide, the same effect is not provided when the wavelength of the converted light is longer than that of the fundamental wave (e.g., a parametric wave or a difference frequency wave). Specifically, if the optical waveguide is designed in accordance with the propagation condition of the converted light, the optical waveguide will be a multiple mode optical waveguide for the fundamental wave, thereby significantly reducing the conversion efficiency.
When the optical waveguide group is asymmetrical along the left-right direction, the beam shape of the emitted light will also be asymmetrical, thereby deteriorating the focusing characteristics. Therefore, it is preferred to provide a structure in which the group of optical waveguides are arranged symmetrically about the center optical waveguide. It is preferred that the number of the optical waveguides is an odd number, so that the center of the power density is localized along the center optical waveguide both for the fundamental wave and the converted light, thereby increasing the overlap between the fundamental wave and the converted light and thus obtaining a high conversion efficiency. For example, when the converted light has a wavelength which is shorter than that of the fundamental wave (e. g., a higher harmonic wave or a sum frequency wave), the optical waveguide group may formed by three optical waveguides having substantially the same propagation direction, with the propagation constant of the center optical waveguide being different from those of the side optical waveguides, whereby phase matching is achieved between the fundamental wave which propagates through the optical waveguide group in a single mode and the converted light which propagates through the center optical waveguide. When the wavelength of the converted light is longer than that of the fundamental wave (e.g., a parametric wave or a difference frequency wave), the optical waveguide group may be formed such that phase matching is achieved between the converted light which propagates through a single mode and the fundamental wave which propagates through the center optical waveguide. It is expected that as the number of optical waveguides provided is increased, the overall size of the optical waveguide group increases, thereby reducing the power density and thus the conversion efficiency. Therefore, the optical waveguide group preferably includes three optical waveguides, i.e., as few optical waveguides as possible with which a symmetrical structure can be obtained.
Moreover, it is possible to adjust the beam shape and/or to improve the coupling efficiency by increasing or decreasing the number of optical waveguides in at least one of an area near the input side of the optical waveguide group and an area near the output side of the optical waveguide group.
It has been reported in the prior art that the optical damage resistance may be improved in the proton exchange layer by providing a proton exchange layer on an LiNbO3 substrate or an LiTaO3 substrate. However, a study conducted by the present inventors has shown that the optical damage resistance of an LiNbO3 substrate subjected to a proton exchange process is not sufficient. For example, while an LiNbO3 substrate with a proton exchange process exhibits an optical damage resistance which is greater than that of an LiNbO3 substrate without a proton exchange process by about an order of magnitude, the LiNbO3 substrate with a proton exchange process exhibits an optical damage resistance which is lower than that of an Mg-doped LiNbO3 substrate without a proton exchange process by a factor of 1 to 10. This is believed to be because the optical damage resistance which has been increased by the doping material is reduced by a proton exchange process. Thus, the present inventors have directed attention to the relationship between the optical damage resistance and the Li concentration in the proton exchange layer. As shown in Embodiment 9 to be described below, an optical waveguide is provided through a proton exchange process on the surface of an LiNbxTa1xe2x88x92xO3 (0xe2x89xa6xxe2x89xa61) substrate, while setting the molarity percentage of Li to 40 mol % or more. Alternatively, an optical waveguide is provided through a proton exchange process on the surface of an LiNbxTa1xe2x88x92xO3 (0xe2x89xa6xxe2x89xa61) substrate with a metal element such as Mg, Zn, Sc or In being added to at least near the surface of the substrate, while setting the molarity percentage Y of Li and the molarity Z of the metal element so as to satisfy the relationship Y+Zxe2x89xa745 mol %. Thus, it is possible to reduce the crystal defect of the proton exchange layer and to improve the optical damage resistance of the optical waveguide.
The coherent light generation apparatus of the present invention uses the optical wavelength conversion device of the present invention which has a desirable optical damage resistance, thereby realizing a high output and stable output characteristics.
Moreover, the optical system of the present invention uses the coherent light generation apparatus of the present invention having reduced small output variations and a high output, whereby it is possible to reduce noise.
Thus, the invention described herein makes possible the advantages of (1) providing an optical waveguide which has a desirable optical damage resistance and is capable of outputting a stable second harmonic wave over a long period of time, thereby realizing an optical wavelength conversion device with a large overlap between the fundamental wave and the higher harmonic wave; (2) providing an optical wavelength conversion device using such an optical waveguide; (3) providing a short wavelength light generation apparatus and an optical information processing apparatus using such an optical wavelength conversion device; (4) providing a method for producing the same; (5) providing an optical wavelength conversion device, a coherent light generation apparatus, and an optical system having a desirable optical damage resistance; and (6) providing an optical waveguide-type optical wavelength conversion device, a coherent light generation apparatus, and an optical system capable of performing an efficient wavelength conversion from a fundamental wave into a parametric wave, a sum frequency wave, a difference frequency wave, and the like, and generating wavelength-converted light having desirable focusing characteristics by improving the beam characteristics thereof.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.